Attempts will be made to establish the mechanism of orthophosphate deinhibition of brain hexokinase. Preliminary experiments suggest that glucose-6-P and phosphate bind to monomeric hexokinase, molecular weight 97.000, with a stoichiometery of one. Both phosphate and ATP are competitive ligands with respect to glucose-6-P; however, these compounds do not affect each other's association with the enzyme. These observations will be explored further. We have recently observed that the brain hexokinase reaction may be reversed. Attempts will be made to study the effects of phosphate on the reverse reaction. An evaluation of the kinetic mechanism of the reverse reaction may be useful in understanding the kinetic mechanism and the effect of phosphate on it.